Nitride-based semiconductor element and method of forming nitride-based semiconductor

ABSTRACT

A nitride-based semiconductor element having superior mass productivity and excellent element characteristics is obtained. This nitride-based semiconductor element comprises a substrate comprising a surface having projection portions, a mask layer formed to be in contact with only the projection portions of the surface of the substrate, a first nitride-based semiconductor layer formed on recess portions of the substrate and the mask layer and a nitride-based semiconductor element layer, formed on the first nitride-based semiconductor layer, having an element region. Thus, the first nitride-based semiconductor layer having low dislocation density is readily formed on the projection portions of the substrate and the mask layer through the mask layer serving for selective growth. When the nitride-based semiconductor element layer having the element region is grown on the first nitride-based semiconductor layer having low dislocation density, a nitride-based semiconductor element having excellent element characteristics can be readily obtained. The first nitride-based semiconductor layer is formed through only single growth on the substrate, whereby a nitride-based semiconductor element having excellent mass productivity is obtained.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a nitride-based semiconductorelement and a method of forming a nitride-based semiconductor, and morespecifically, it relates to a nitride-based semiconductor elementincluding a nitride-based semiconductor layer formed by epitaxiallateral overgrowth and a method of forming a nitride-basedsemiconductor.

[0003] 2. Description of the Prior Art

[0004] In recent years, a nitride-based semiconductor element utilizinga group III nitride-based semiconductor is actively developed as asemiconductor element employed for a semiconductor light-emitting devicesuch as a light-emitting diode device or a semiconductor laser device oran electronic device such as a transistor. In order to fabricate such anitride-based semiconductor element, a nitride-based semiconductor layeris epitaxially grown on a substrate consisting of sapphire or the like.

[0005] In this case, the substrate of sapphire or the like and thenitride-based semiconductor layer have different lattice constants andhence the nitride-based semiconductor layer grown on the substrate ofsapphire or the like has dislocations (lattice defects) verticallyextending from the substrate to the surface of the semiconductor layer.Such dislocations in the nitride-based semiconductor layer result indeterioration of the element characteristics of the semiconductorelement and reduction of the reliability thereof.

[0006] As a method of reducing the density of the aforementioneddislocations in the nitride-based semiconductor layer, epitaxial lateralgrowth is generally proposed. This epitaxial lateral growth is disclosedin International Workshop on Nitride Semiconductors -IWN2000-, Nagoya,Japan, 2000, p. 79, for example.

[0007] FIGS. 29 to 33 are sectional views for illustrating aconventional method of forming a nitride-based semiconductor employingepitaxial lateral overgrowth. The conventional method of forming anitride-based semiconductor employing epitaxial lateral overgrowth isnow described with reference to FIGS. 29 to 33.

[0008] First, a GaN layer 102 for serving as an underlayer is formed ona substrate 101 consisting of sapphire or SiC, as shown in FIG. 29.Then, mask layers 103 are formed on prescribed regions of the GaN layer102.

[0009] Then, portions of the GaN layer 102 located under regions formedwith no mask layers 103 are removed by etching while etching thesubstrate 101 by a thickness in the range not reaching the bottomsurface thereof through the mask layers 103 serving for etching in thisprocess. Thus, the substrate 101 is brought into a ridged shape, whilestripe-patterned GaN layers 102 to be in contact substantially with theoverall upper surfaces of projection potions of the substrate 101, asshown in FIG. 30.

[0010] Then, undoped GaN layers 104 are re-grown from exposed sidesurfaces, serving as seed crystals, of the GaN layers 102, as shown inFIG. 31. The undoped GaN layers 104 are laterally grown in an initialstage. From the state shown in FIG. 31, the undoped GaN layers 104 aregrown upward while laterally growing on the mask layers 103 serving forselective growth in this process, as shown in FIG. 32. At this time,voids 105 are formed between the undoped GaN layers 104 and the bottomsurfaces of recess portions of the substrate 101. The undoped GaN layers104 laterally growing on the mask layers 103 coalesce into a continuousundoped GaN layer 104 having a flattened surface, as shown in FIG. 33.

[0011] In the conventional method of forming a nitride-basedsemiconductor, as hereinabove described, the undoped GaN layer 102 isformed by epitaxial lateral overgrowth from the exposed side surfaces ofthe GaN layers 102 serving as seed crystals, whereby lattice defects arescarcely propagated from the GaN layers 102 to a portion around thesurface of the undoped GaN layer 104. Thus, the undoped GaN layer 104reduced in dislocation density is obtained. When a nitride-basedsemiconductor element layer (not shown) having an element region isformed on such an undoped GaN layer 104 reduced in dislocation density,a nitride-based semiconductor element having excellent crystallinity canbe formed.

[0012] In the aforementioned conventional method of forming anitride-based semiconductor employing epitaxial lateral overgrowth,however, the substrate 101 is brought into the ridged shape by removingthe portions of the GaN layers 102 located under the regions formed withno mask layers 103 by etching and thereafter further etching thesubstrate 101. In general, therefore, the layers GaN 102, which arehardly etched nitride-based semiconductor layers, must be etched alongthe overall thicknesses thereof while the surface of the substrate 101must also be etched. Thus, the etching time for bringing the substrate101 into the ridged shape is disadvantageously increased. Consequently,the nitride-based semiconductor is disadvantageously reduced in massproductivity.

[0013] In the aforementioned conventional method of forming anitride-based semiconductor employing epitaxial lateral overgrowth,further, the undoped GaN layer 104 is formed by growing the GaN layers102 serving as underlayers on the substrate 101 and thereafterepitaxially laterally overgrowing the GaN layers 102. Therefore, thismethod requires two crystal growth steps for the GaN layers 102 and theundoped GaN layer 104. In general, therefore, the nitride-basedsemiconductor is reduced in mass productivity also in this point.

SUMMARY OF THE INVENTION

[0014] An object of the present invention is to provide a nitride-basedsemiconductor element having superior mass productivity and excellentelement characteristics.

[0015] Another object of the present invention is to provide a method offorming a nitride-based semiconductor capable of obtaining anitride-based semiconductor layer having excellent mass productivity andlow dislocation density.

[0016] A nitride-based semiconductor element according to a first aspectof the present invention comprises a substrate comprising a surfacehaving projection portions, a mask layer formed to be in contact withonly the projection portions of the surface of the substrate, a firstnitride-based semiconductor layer formed on recess portions of thesubstrate and the mask layer, and a nitride-based semiconductor elementlayer, formed on the first nitride-based semiconductor layer, having anelement region.

[0017] The nitride-based semiconductor element according to the firstaspect is provided with the substrate comprising a surface havingprojection portions and the mask layer formed to be in contact with onlythe projection portions of the surface of the substrate as hereinabovedescribed, whereby the first nitride-based semiconductor layer havinglow dislocation density can be readily formed on the recess potions ofthe substrate and the mask layer through the mask layer serving forselective growth. When the nitride-based semiconductor element layerhaving the element region is grown on the first nitride-basedsemiconductor layer having low dislocation density, a nitride-basedsemiconductor element having excellent element characteristics can bereadily obtained. Further, only the surface of the substrate may beetched for forming the projection portions. Thus, the etching time forforming the projection portions can be reduced. According to the firstaspect, further, the first nitride-based semiconductor layer can beformed through single growth on the substrate. Consequently, anitride-based semiconductor element having excellent mass productivitycan be obtained.

[0018] In the aforementioned nitride-based semiconductor elementaccording to the first aspect, the substrate preferably includes asubstrate selected from a group consisting of a sapphire substrate, aspinel substrate, an Si substrate, an SiC substrate, a GaN substrate, aGaAs substrate, a GaP substrate, an InP substrate, a ZrB₂ substrate anda quartz substrate. In this case, the substrate preferably includes asapphire substrate, while the mask layer and the projection portions ofthe surface of the substrate are preferably formed in the shape ofstripes being parallel to the [1-100] direction of the sapphiresubstrate. The substrate preferably includes an Si substrate, and themask layer and the projection portions of the surface of the substrateare preferably formed in the shape of stripes being parallel to the[1-10] direction of the Si substrate.

[0019] The aforementioned nitride-based semiconductor element accordingto the first aspect preferably further comprises a buffer layer formedon the interface between the recess portions of the substrate and thefirst nitride-based semiconductor layer. According to this structure,the first nitride-based semiconductor layer having lower dislocationdensity can be formed on the buffer layer.

[0020] A nitride-based semiconductor element according to a secondaspect of the present invention comprises an underlayer, formed on asubstrate, consisting of a nitride-based semiconductor and comprising asurface having projection portions, a mask layer formed to be in contactwith only the projection portions of the surface of the underlayer, afirst nitride-based semiconductor layer formed on recess portions of theunderlayer and the mask layer, and a nitride-based semiconductor elementlayer, formed on the first nitride-based semiconductor layer, having anelement region.

[0021] The nitride-based semiconductor element according to the secondaspect is provided with the underlayer comprising the surface having theprojection portions and the mask layer formed to be in contact with onlythe projection portions of the surface of the underlayer as describedabove, whereby the first nitride-based semiconductor layer having lowdislocation density can be readily formed on the recess portions of theunderlayer and the mask layer through the mask layer serving forselective growth. When the nitride-based semiconductor element layerhaving the element region is grown on the first nitride-basedsemiconductor layer having low dislocation density, a nitride-basedsemiconductor element having excellent element characteristics can bereadily obtained. Only the surface of the underlayer consisting of anitride-based semiconductor may be etched for forming the projectionportions on the surface. Thus, the etching time for forming theprojection portions on the surface can be reduced, and a nitride-basedsemiconductor element having excellent mass productivity can be obtainedas a result.

[0022] The aforementioned nitride-based semiconductor element accordingto the second aspect preferably further comprises a buffer layer formedbetween the substrate and the underlayer. According to this structure,the underlayer consisting of a nitride-based semiconductor having lowdislocation density can be readily formed on the buffer layer.

[0023] In the aforementioned nitride-based semiconductor elementaccording to the second embodiment, the substrate preferably includes asubstrate selected from a group consisting of a sapphire substrate, aspinel substrate, an Si substrate, an SiC substrate, a GaAs substrate, aGaP substrate, an InP substrate, a ZrB₂ substrate and a quartzsubstrate.

[0024] In the aforementioned nitride-based semiconductor elementaccording to the second aspect, the underlayer preferably includes a GaNlayer, and the mask layer and the projection portions of the surface ofthe underlayer are preferably formed in the shape of stripes beingparallel to the [11-20] direction or the [1-100] direction of the GaNlayer.

[0025] A method of forming a nitride-based semiconductor according to athird aspect of the present invention comprises steps of formingprojection portions on a surface on a substrate, forming a mask layer tobe in contact with only the projection portions of the surface of thesubstrate and growing a first nitride-based semiconductor layer onrecess portions of the substrate and the mask layer through the masklayer serving for selective growth.

[0026] In the method of forming a nitride-based semiconductor accordingto the third embodiment, the surface having the projection portions isformed on the substrate while the mask layer is formed to be in contactwith only the projection portions of the surface of the substrate,whereby the first nitride-based semiconductor layer having lowdislocation density can be readily formed on the recess portions of thesubstrate and the mask layer when grown through the mask layer servingfor selective growth. Further, only the surface of the substrate may beetched for forming the projection portions on the surface. Thus, theetching time for forming the projection portions on the surface can bereduced. According to the third aspect, further, the first nitride-basedsemiconductor layer can be formed through single growth on thesubstrate. Consequently, a method of forming a nitride-basedsemiconductor excellent in mass productivity can be obtained.

[0027] The aforementioned method of forming a nitride-basedsemiconductor according to the third aspect preferably further comprisesa step of forming a buffer layer on the recess portions of the substratein advance of the step of growing the first nitride-based semiconductorlayer. According to this structure, the first nitride-basedsemiconductor layer having lower dislocation density can be formed onthe buffer layer.

[0028] In the aforementioned method of forming a nitride-basedsemiconductor according to the third aspect, the steps of forming theprojection portions on the surface on the substrate and forming the masklayer preferably include a step of forming the mask layer on the surfaceof the substrate and thereafter etching the surface of the substratethrough the mask layer serving for etching in this step therebysimultaneously forming the projection portions on the surface of thesubstrate and the mask layer coming into contact with only theprojection portions of the surface. According to this method, the masklayer serving for etching simultaneously serves for selective growth,whereby the fabrication process can be simplified.

[0029] The aforementioned method of forming a nitride-basedsemiconductor according to the third aspect preferably further comprisesa step of growing a nitride-based semiconductor element layer having anelement region on the first nitride-based semiconductor layer. Accordingto this structure, the nitride-based semiconductor element layer havingthe element region can be grown on the first nitride-based semiconductorlayer having low dislocation density, whereby a nitride-basedsemiconductor element having excellent element characteristics can bereadily formed.

[0030] In the aforementioned method of forming a nitride-basedsemiconductor according to the third aspect, the substrate preferablyincludes a substrate selected from a group consisting of a sapphiresubstrate, a spinel substrate, an Si substrate, an SiC substrate, a GaNsubstrate, a GaAs substrate, a GaP substrate, an InP substrate, a ZrB₂substrate and a quartz substrate. In this case, the substrate preferablyincludes a sapphire substrate, and the mask layer and the projectionportions of the surface of the substrate are preferably formed in theshape of stripes being parallel to the [1-100] direction of the sapphiresubstrate. Alternatively, the substrate preferably includes an Sisubstrate, and the mask layer and the projection portions of the surfaceof the substrate are preferably formed in the shape of stripes beingparallel to the [1-10] direction of the Si substrate.

[0031] A method of forming a nitride-based semiconductor according to afourth aspect of the present invention comprises steps of forming anunderlayer consisting of a nitride-based semiconductor on a substrate,forming projection portions on a surface on the underlayer, forming amask layer to be in contact with only the projection portions of thesurface of the underlayer and growing a first nitride-basedsemiconductor layer on recess portions of the underlayer and the masklayer through the mask layer serving for selective growth.

[0032] In the method of forming a nitride-based semiconductor accordingto the fourth aspect, the surface having the projection portions isformed on the underlayer consisting of a nitride-based semiconductorlayer provided on the substrate while the mask layer is formed to be incontact with only the projection portions of the surface of theunderlayer, whereby the first nitride-based semiconductor layer havinglow dislocation density can be readily formed on the recess portions ofthe underlayer and the mask layer when grown through the mask layerserving for selective growth. Further, only the surface of theunderlayer consisting of a nitride-based semiconductor may be etched forforming the projection portions on the surface. Thus, the etching timefor forming the projection portions on the surface can be reduced, and amethod of forming a nitride-based semiconductor having excellent massproductivity can be provided as a result.

[0033] The aforementioned method of forming a nitride-basedsemiconductor according to the fourth aspect preferably furthercomprises a step of forming a buffer layer on the substrate in advanceof the step of forming the underlayer consisting of a nitride-basedsemiconductor. According to this structure, the underlayer consisting ofa nitride-based semiconductor having low dislocation density can bereadily formed on the buffer layer.

[0034] In the aforementioned method of forming a nitride-basedsemiconductor according to the fourth aspect, the steps of forming theprojection portions on the surface on the underlayer and forming themask layer preferably include a step of forming the mask layer on thesurface of the underlayer and thereafter etching the surface of theunderlayer through the mask layer serving for etching in this stepthereby simultaneously forming the projection portions on the surface ofthe underlayer and the mask layer coming into contact with only theprojection portions of the surface. According to this method, the masklayer serving for etching simultaneously serves for selective growth,whereby the fabrication process can be simplified.

[0035] The aforementioned method of forming a nitride-basedsemiconductor according to the fourth aspect preferably furthercomprises a step of growing a nitride-based semiconductor element layerhaving an element region on the first nitride-based semiconductor layer.According to this structure, the nitride-based semiconductor elementlayer having the element region can be grown on the first nitride-basedsemiconductor layer having low dislocation density, whereby anitride-based semiconductor element having excellent elementcharacteristics can be readily formed.

[0036] In the aforementioned method of forming a nitride-basedsemiconductor according to the fourth aspect, the substrate preferablyincludes a substrate selected from a group consisting of a sapphiresubstrate, a spinel substrate, an Si substrate, an SiC substrate, a GaAssubstrate, a GaP substrate, an InP substrate, a ZrB₂ substrate and aquartz substrate.

[0037] In the aforementioned method of forming a nitride-basedsemiconductor according to the fourth aspect, the underlayer preferablyincludes a GaN layer, and the mask layer and the projection portions ofthe surface of the underlayer are preferably formed in the shape ofstripes being parallel to the [11-20] direction or the [1-100] directionof the GaN layer.

[0038] The foregoing and other objects, features, aspects and advantagesof the present invention will become more apparent from the followingdetailed description of the present invention when taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] FIGS. 1 to 6 are sectional views for illustrating a method offorming a nitride-based semiconductor according to a first embodiment ofthe present invention;

[0040]FIG. 7 is a perspective view showing a semiconductor laser devicefabricated with the method of forming a nitride-based semiconductoraccording to the first embodiment of the present invention;

[0041] FIGS. 8 to 13 are sectional views for illustrating a method offorming a nitride-based semiconductor according to a second embodimentof the present invention;

[0042]FIG. 14 is a perspective view showing a semiconductor laser devicefabricated with the method of forming a nitride-based semiconductoraccording to the second embodiment of the present invention;

[0043] FIGS. 15 to 20 are sectional views for illustrating a method offorming a nitride-based semiconductor according to a third embodiment ofthe present invention;

[0044]FIG. 21 is a perspective view showing a semiconductor laser devicefabricated with the method of forming a nitride-based semiconductoraccording to the third embodiment of the present invention;

[0045] FIGS. 22 to 27 are sectional views for illustrating a method offorming a nitride-based semiconductor according to a fourth embodimentof the present invention;

[0046]FIG. 28 is a perspective view showing a semiconductor laser devicefabricated with the method of forming a nitride-based semiconductoraccording to the fourth embodiment of the present invention; and

[0047] FIGS. 29 to 33 are sectional views for illustrating aconventional method of forming a nitride-based semiconductor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0048] Embodiments of the present invention are now described withreference to the drawings.

[0049] (First Embodiment)

[0050] A method of forming a nitride-based semiconductor according to afirst embodiment of the present invention is described with reference toFIGS. 1 to 6.

[0051] First, striped mask layers 2 of SiO₂ having a thickness of about0.5 μm are formed on a sapphire (0001) plane substrate 1 (hereinafterreferred to as “sapphire substrate 1”), as shown in FIG. 1. The stripepatterns of the mask layers 2 are formed in a cycle of about 7 μm sothat the width of the mask layers 2 is about 5 μm and the intervalbetween each adjacent pair of mask layers 2 (the width of mask openings)is about 2 μm. The striped mask layers 2 are formed in parallel to the[1-100] direction of the sapphire substrate 1. The sapphire substrate 1is an example of the “substrate” according to the present invention.

[0052] The mask layers 2 are employed as masks for etching the surfaceof the sapphire substrate 1 by a thickness of about 1 μm through RIE(reactive ion etching) or the like. Thus, the surface of the sapphiresubstrate 1 is brought into an uneven shape, as shown in FIG. 2. Theshape of the projection portions varies with etching conditions, suchthat upper parts of recess portions may be larger or smaller in widththan bottom parts thereof. In the following description, side surfacesof the recess portions of the etched sapphire substrate 1 are nearlyperpendicular to the upper surface of projection portions of thesapphire substrate 1. The uneven surface of the sapphire substrate 1 hasridges of a height of about 1 μm and grooves of a terrace width of about2 μm, and they are formed in parallel to the [1-100] direction of thesapphire substrate 1.

[0053] Then, low-temperature buffer layers 3 of GaN having a thicknessof about 15 nm are grown by a crystal growth method such as MOVPE (metalorganic vapor phase epitaxy) to be in contact substantially with theoverall bottom surfaces of the recess portions of the sapphire substrate1, as shown in FIG. 3. In this case, the low-temperature buffer layers 3are hardly formed on the mask layers 2 consisting of SiO₂.Alternatively, the low-temperature buffer layers 3 may be formed notonly on the bottom surfaces but also on the side surfaces of the recessportions. The low-temperature buffer layers 3 may not be formed on theoverall bottom surfaces of the recess portions but may be partiallyformed on the bottom surfaces of the recess portions. Thelow-temperature buffer layers 3 are examples of the “buffer layer”according to the present invention.

[0054] Then, undoped GaN layers 4 are grown on the low-temperaturebuffer layers 3 consisting of GaN. In this case, the low-temperaturebuffer layers 3 and the undoped GaN layers 4 are successively grownwithout taking the sapphire substrate 1 out of the growth apparatus. Inan initial stage, the undoped GaN layers 4 are vertically (upwardly)grown on the low-temperature buffer layers 3. If the low-temperaturebuffer layers 3 are formed on the side surfaces of the recess portions,the undoped GaN layers 4 are grown laterally from the low-temperaturebuffer layers 3 formed on the side surfaces. When this growth is furthercontinued, undoped GaN layers 4 having facets on side surfaces thereofare formed on the recess portions, as shown in FIG. 4. The undoped GaNlayers 4 are examples of the “first nitride-based semiconductor layer”according to the present invention.

[0055] From the state shown in FIG. 4, the undoped GaN layers 4 arelaterally grown on the mask layers 2, as shown in FIG. 5. The undopedGaN layers 4 laterally grown on the mask layers 2 coalesce into acontinuous undoped GaN layer 4 of about 5 μm in thickness having aflattened surface, as shown in FIG. 6.

[0056] In the method of forming a nitride-based semiconductor accordingto the first embodiment, the undoped GaN layers 4 are grown from therecess portions of the sapphire substrate 1 as hereinabove described,whereby dislocations of the undoped GaN layers 4 are bent in thein-plane direction of the (0001) plane of the undoped GaN layers 4 whenthe undoped GaN layers 4 are laterally grown from the low-temperaturebuffer layers 3 formed on the side surfaces of the recess portions or onthe mask layers 2. Thus, the dislocation density can be reduced aroundthe surfaces of the undoped GaN layers 4.

[0057] In the method of forming a nitride-based semiconductor accordingto the first embodiment, the surface of the sapphire substrate 1 isbrought into an uneven shape as hereinabove described, whereby only thesurface of the sapphire substrate 1 may be etched through the masklayers 2 serving for etching. Thus, the etching time for forming theprojection portions on the surface on the sapphire substrate 1 can bereduced as compared with the conventional process of forming theprojection portions on the surface shown in FIG. 30. Consequently, amethod of forming a nitride-based semiconductor having excellent massproductivity can be obtained.

[0058] In the method of forming a nitride-based semiconductor accordingto the first embodiment, growth of the low-temperature buffer layers 3of GaN formed on the recess portions of the sapphire substrate 1 andselective growth of the undoped GaN layer 4 are successively performedwithout taking the sapphire substrate 1 out of the growth apparatus, ashereinabove described. Thus, the undoped GaN layer 4 having lowdislocation density can be formed through a single growth step. A methodof forming a nitride-based semiconductor having excellent massproductivity can be obtained also in this point.

[0059] In the method of forming a nitride-based semiconductor accordingto the first embodiment, further, the undoped GaN layer 4 is grown onthe low-temperature buffer layers 3 provided on the sapphire substrate1, whereby the undoped GaN layer 4 can be grown in lower dislocationdensity as compared with that directly grown on the sapphire substrate1.

[0060]FIG. 7 is a perspective view showing a semiconductor laser devicefabricated with the aforementioned method of forming a nitride-basedsemiconductor according to the first embodiment. The structure of thesemiconductor laser device fabricated with the method of forming anitride-based semiconductor according to the first embodiment is nowdescribed with reference to FIG. 7.

[0061] In the structure of the semiconductor laser device according tothe first embodiment, an n-type contact layer 5 of n-type GaN having athickness of about 4 μm is formed on the undoped GaN layer 4 accordingto the first embodiment shown in FIG. 6, as shown in FIG. 7. Ananti-cracking layer 6 of n-type AlGaInN having a thickness of about 0.1μm, an n-type second cladding layer 7 of n-type AlGaN having a thicknessof about 0.45 μm, an n-type first cladding layer 8 of n-type GaN havinga thickness of about 50 nm (about 0.05 μm) and a multiple quantum well(MQW) emission layer 9 of GaInN are successively formed on the n-typecontact layer 5. The MQW emission layer 9 is formed by alternatelystacking five undoped GaN barrier layers of about 4 nm in thickness andfour compressive strain undoped GaInN well layers of about 4 nm inthickness.

[0062] A p-type first cladding layer 10 of p-type GaN having a thicknessof about 40 nm (about 0.04 μm) is formed on the MQW emission layer 9. Amesa (trapezoidal) p-type second cladding layer 11 of p-type AlGaNhaving a height of about 0.45 μm is formed on the p-type first claddinglayer 10. Current blocking layers 12 of n-type GaN having a thickness ofabout 0.2 μm are formed to cover regions on the p-type first claddinglayer 10 other than that formed with the p-type second cladding layer 11and the side surfaces of the mesa p-type second cladding layer 11 whileexposing the upper surface of the p-type second cladding layer 11. Ap-type contact layer 13 of p-type GaN having a thickness of about 3 μmto about 5 μm is formed on the current blocking layers 12 to be incontact with the exposed upper surface of the p-type second claddinglayer 11.

[0063] The layers from the p-type contact layer 13 to the ntype contactlayer 5 are partially removed. Protective films 14 of an insulator suchas SiO₂ or SiN are formed to cover parts of the exposed surface of then-type contact layer 5 and the exposed side surfaces of theanti-cracking layer 6, the n-type second cladding layer 7, the n-typefirst cladding layer 8, the MQW emission layer 9, the ptype firstcladding layer 10, the current blocking layers 12 and the p-type contactlayer 13.

[0064] A p-side electrode 15 is formed on the upper surface of thep-type contact layer 13 while an n-side electrode 16 is formed on thepartially removed and exposed surface of the n-type contact layer 5.

[0065] The n-type contact layer 5, the anti-cracking layer 6, the n-typesecond cladding layer 7, the n-type first cladding layer 8, the MQWemission layer 9, the p-type first cladding layer 10, the p-type secondcladding layer 11, the current blocking layers 12 and the n-type contactlayer 13 are examples of the “nitride-based semiconductor element layerhaving an element region” according to the present invention.

[0066] In the semiconductor laser device according to the firstembodiment, the undoped GaN layer 4 having excellent mass productivityand low dislocation density formed by the method of forming anitride-based semiconductor according to the first embodiment shown inFIG. 1 to 6 is employed as an underlayer for forming the layers 5 to 13thereon as hereinabove described, whereby excellent crystallinity can beimplemented in the layers 5 to 13. Consequently, a semiconductor laserdevice having excellent mass productivity and excellent devicecharacteristics can be obtained according to the first embodiment.

[0067] (Second Embodiment)

[0068] Referring to FIGS. 8 to 13, an n-type Si (111) plane substrate 21(hereinafter referred to as “Si substrate 21”) having conductivity isemployed in a second embodiment of the present invention in place of theinsulating sapphire substrate 1 in the first embodiment. A method offorming a nitride-based semiconductor according to the second embodimentis now described with reference to FIGS. 8 to 13.

[0069] According to the second embodiment, striped mask layers 22 ofSiO₂ having a thickness of about 0.5 μm are formed on the n-type Sisubstrate 21, as shown in FIG. 8. The stripe patterns of the mask layers22 are formed in a cycle of about 7 μm, so that the mask layers 22 areabout 5 μm in width and the interval between each adjacent pair of masklayers 22 (the width of mask openings) is about 2 μm. The striped masklayers 22 are formed in parallel to the [1-10] direction of the Sisubstrate 21. The Si substrate 21 is an example of the “substrate”according to the present invention.

[0070] The mask layers 22 are employed as masks for etching the surfaceof the Si substrate 21 by a thickness of about 1 μm through wet etchingor the like. Thus, the surface of the Si substrate 21 is brought into anuneven shape, as shown in FIG. 9. The shape of the projection portionsvaries with etching conditions, such that upper parts of recess portionsmay be larger or smaller in width than bottom parts thereof. In thefollowing description, projection portions of the etched Si substrate 21are in the form of a mesa (trapezoid). The uneven surface of the Sisubstrate 21 has ridges of a height of about 1 μm, and they are formedin parallel to the [1-10] direction of the Si substrate 21.

[0071] Then, buffer layers 23 of Si-doped AlGaN having a thickness ofabout 15 nm are grown by a crystal growth method such as MOVPE to be incontact substantially with the overall bottom surfaces of the recessportions of the Si substrate 21, as shown in FIG. 10. In this case, thebuffer layers 23 are hardly formed on the mask layers 22 consisting ofSiO₂. Alternatively, the buffer layers 23 may be formed not only on thebottom surfaces but also on the side surfaces of the recess portions.The buffer layers 23 may not be formed on the overall bottom surfaces ofthe recess portions but may be partially formed on the bottom surfacesof the recess portions.

[0072] Then, Si-doped GaN layers 24 are grown on the buffer layers 23consisting of Si-doped AlGaN. In this case, the buffer layers 23 and theSi-doped GaN layers 24 are successively grown without taking the Sisubstrate 21 out of the growth apparatus. In an initial stage, theSi-doped GaN layers 24 are vertically (upwardly) grown on the bufferlayers 23. If the buffer layers 23 are formed on the side surfaces ofthe recess portions, the Si-doped GaN layers 24 are grown laterally fromthe buffer layers 23 formed on the side surfaces. When this growth isfurther continued, Si-doped GaN layers 24 having facets on side surfacesthereof are formed on the recess portions, as shown in FIG. 11. TheSi-doped GaN layers 24 are examples of the “first nitride-basedsemiconductor layer” according to the present invention.

[0073] From the state shown in FIG. 11, the Si-doped GaN layers 24 arelaterally grown on the mask layers 22, as shown in FIG. 12. The Si-dopedGaN layers 24 laterally grown on the mask layers 22 coalesce into acontinuous Si-doped GaN layer 24 of about 5 pm in thickness having aflattened surface, as shown in FIG. 13.

[0074] In the method of forming a nitride-based semiconductor accordingto the second embodiment, the Si-doped GaN layers 24 are grown from therecess portions of the Si substrate 21, whereby dislocations of theSi-doped GaN layers 24 are bent in the in-plane direction of the (0001)plane of the Si-doped GaN layers 24 when the Si-doped GaN layers 24 arelaterally grown from the buffer layers 23 formed on the side surfaces ofthe recess portions or on the mask layers 22. Thus, the dislocationdensity can be reduced around the surfaces of the Si-doped GaN layers24.

[0075] In the method of forming a nitride-based semiconductor accordingto the second embodiment, the surface of the Si substrate 21 is broughtinto an uneven shape similarly to the first embodiment, whereby only thesurface of the Si substrate 21 may be etched through the mask layers 22serving for etching. Thus, the etching time for forming the projectionportions on the surface on the Si substrate 21 can be reduced ascompared with the conventional process of forming the projectionportions on the surface shown in FIG. 30. Consequently, a method offorming a nitride-based semiconductor having excellent mass productivitycan be obtained.

[0076] In the method of forming a nitride-based semiconductor accordingto the second embodiment, growth of the buffer layers 23 of Si-dopedAlGaN formed on the Si substrate 21 and selective growth of the Si-dopedGaN layer 24 are successively performed without taking the Si substrate21 out of the growth apparatus, similarly to the first embodiment. Thus,the Si-doped GaN layer 24 having low dislocation density can be formedthrough a single growth step. A method of forming a nitride-basedsemiconductor having excellent mass productivity can be obtained also inthis point.

[0077] In the method of forming a nitride-based semiconductor accordingto the second embodiment, further, the Si-doped GaN layer 24 is grown onthe buffer layers 23 provided on the Si substrate 21, whereby theSi-doped GaN layer 24 can be grown in lower dislocation density ascompared with that directly grown on the Si substrate 21.

[0078]FIG. 14 is a perspective view showing a semiconductor laser devicefabricated with the aforementioned method of forming a nitride-basedsemiconductor according to the second embodiment. The structure of thesemiconductor laser device fabricated with the method of forming anitride-based semiconductor according to the second embodiment is nowdescribed with reference to FIG. 14.

[0079] In the structure of the semiconductor laser device according tothe second embodiment, an anti-cracking layer 25 of n-type AlGaInNhaving a thickness of about 0.1 μM, an n-type second cladding layer 26of n-type AlGaN having a thickness of about 0.45 μm, an n-type firstcladding layer 27 of n-type GaN having a thickness of about 50 nm (about0.05 μm) and a multiple quantum well (MQW) emission layer 28 of GaInNare successively formed on the Si-doped GaN layer 24 according to thesecond embodiment shown in FIG. 13, as shown in FIG. 14. The MQWemission layer 28 is formed by alternately stacking five undoped GaNbarrier layers of about 4 nm in thickness and four compressive strainundoped GaInN well layers of about 4 nm in thickness.

[0080] A p-type first cladding layer 29 of p-type GaN having a thicknessof about 40 nm (about 0.04 μm) is formed on the MQW emission layer 28. Amesa (trapezoidal) p-type second cladding layer 30 of p-type AlGaNhaving a height of about 0.45 μm is formed on the p-type first claddinglayer 29. Current blocking layers 31 of n-type GaN having a thickness ofabout 0.2 μm are formed to cover regions on the p-type first claddinglayer 29 other than that formed with the p-type second cladding layer 30and the side surfaces of the mesa p-type second cladding layer 30 whileexposing the upper surface of the p-type second cladding layer 30. Ap-type contact layer 32 of p-type GaN having a thickness of about 3 μmto about 5 μm is formed on the current blocking layers 31 to be incontact with the exposed upper surface of the p-type second claddinglayer 30.

[0081] A p-side electrode 33 is formed on a projection portion of thep-type contact layer 32 reflecting the mesa shape of the p-type secondcladding layer 30. According to the second embodiment, the Si substrate21 has conductivity dissimilarly to the sapphire substrate 1 accordingto the first embodiment, and hence an n-side electrode 34 is formed onthe back surface of the Si substrate 21.

[0082] The anti-cracking layer 25, the n-type second cladding layer 26,the n-type first cladding layer 27, the MQW emission layer 28, thep-type first cladding layer 29, the p-type second cladding layer 30, thecurrent blocking layers 31 and the n-type contact layer 32 are examplesof the “nitride-based semiconductor element layer having an elementregion” according to the present invention.

[0083] In the semiconductor laser device according to the secondembodiment, the Si-doped GaN layer 24 having excellent mass productivityand low dislocation density formed by the method of forming anitride-based semiconductor according to the second embodiment shown inFIG. 8 to 13 is employed as an underlayer for forming the layers 25 to32 thereon as hereinabove described, whereby excellent crystallinity canbe implemented in the layers 25 to 32. Consequently, a semiconductorlaser device having excellent mass productivity and excellent devicecharacteristics can be obtained.

[0084] While the sapphire substrate 1 and the Si substrate 21 areemployed in the aforementioned first and second embodimentsrespectively, for example, the present invention is not restricted tothis but a spinel substrate, an SiC substrate, a GaAs substrate, a GaPsubstrate, an InP substrate, a ZrB₂ substrate or a quartz substrate mayalternatively be employed.

[0085] Further alternatively, a GaN substrate may be employed in each ofthe aforementioned first and second embodiments. In this case, thelow-temperature buffer layers 3 or the buffer layers 23 may notnecessarily be formed.

[0086] (Third Embodiment)

[0087] Referring to FIGS. 15 to 20, epitaxial lateral overgrowth isperformed through an underlayer 43 comprising a surface havingprojection portions formed on a sapphire (0001) plane substrate 41(hereinafter referred to as “sapphire substrate 41”) in a thirdembodiment of the present invention. A method of forming a nitridesemiconductor according to the third embodiment is now described indetail with reference to FIGS. 15 to 20.

[0088] First, low-temperature buffer layers 42 of AlGaN having athickness of about 15 nm and the underlayer 43 of undoped GaN having athickness of about 2 μm are formed on the sapphire substrate 41 by acrystal growth method such as MOVPE, as shown in FIG. 15. The sapphiresubstrate 41 is an example of the “substrate” according to the presentinvention. The low-temperature buffer layers 42 are examples of the“buffer layer” according to the present invention.

[0089] Striped mask layers 44 of SiO₂ having a thickness of about 0.5 μmare formed on the underlayer 43. The stripe patterns of the mask layers44 are formed in a cycle of about 6 μm so that the width of the masklayers 44 is about 5 μm and the interval between each adjacent pair ofmask layers 44 (the width of mask openings) is about 1 μm. The stripedmask layers 44 are formed in parallel to the [11-20] direction of theunderlayer 43 consisting of GaN.

[0090] The mask layers 44 are employed as masks for etching the surfaceof the underlayer 43 by a thickness of about 1 μm through RIE or thelike. Thus, the surface of the underlayer 43 is brought into an unevenshape, as shown in FIG. 16. The shape of the projection portions varieswith etching conditions, such that upper parts of recess portions may belarger or smaller in width than bottom parts thereof. In the followingdescription, projection portions of the etched underlayer 43 are in theform of a mesa (trapezoid). The uneven surface of the underlayer 43 hasridges of a height of about 1 μm, and they are formed in parallel to the[11-20] direction of the underlayer 43 consisting of undoped GaN.

[0091] Then, undoped GaN layers 45 are re-grown from the bottom and sidesurfaces, serving as seed crystals, of the exposed recess portions ofthe underlayer 43 consisting of undoped GaN, as shown in FIG. 17. In aninitial stage, the undoped GaN layers 45 are vertically (upwardly) grownfrom the bottom surfaces of the recess portions of the underlayer 43 andalso laterally grown from the side surfaces of the recess portions ofthe underlayer 43, as shown in FIGS. 17 and 18. The undoped GaN layers45 are examples of the “first nitride-based semiconductor layer”according to the present invention.

[0092] From the state shown in FIG. 18, the undoped GaN layers 45 arelaterally grown on the mask layers 44, as shown in FIG. 19. The undopedGaN layers 45 laterally grown on the mask layers 44 coalesce into acontinuous undoped GaN layer 45 of about 5 μm in thickness having aflattened surface, as shown in FIG. 20.

[0093] In the method of forming a nitride-based semiconductor accordingto the third embodiment, the undoped GaN layers 45 are grown from thebottom and side surfaces, serving as seed crystals, of the recessportions of the underlayer 43 consisting of undoped GaN as hereinabovedescribed, whereby dislocations of the undoped GaN layers 45 are bent inthe in-plane direction of the (0001) plane of the undoped GaN layers 45when the undoped GaN layers 45 are laterally grown from the sidesurfaces of the recess portions of the underlayer 43 or on the masklayers 44. Thus, the dislocation density can be reduced around thesurfaces of the undoped GaN layers 45.

[0094] In the method of forming a nitride-based semiconductor accordingto the third embodiment, the surface of the underlayer 43 is broughtinto an uneven shape as hereinabove described, whereby only the surfaceof the underlayer 43 may be etched. Thus, the etching time for formingthe projection portions on the surface on the underlayer 43 can bereduced as compared with the conventional process of forming theprojection portions on the surface shown in FIG. 30. Consequently, amethod of forming a nitride-based semiconductor having excellent massproductivity can be obtained similarly to the first and secondembodiments.

[0095] In the method of forming a nitride-based semiconductor accordingto the third embodiment, the underlayer 43 consisting of undoped GaN isgrown after forming the low-temperature buffer layers 42 on the sapphiresubstrate 41, whereby the underlayer 43 having low dislocation densitycan be readily formed.

[0096]FIG. 21 is a perspective view showing a semiconductor laser devicefabricated with the aforementioned method of forming a nitride-basedsemiconductor according to the third embodiment. The structure of thesemiconductor laser device fabricated with the method of forming anitride-based semiconductor according to the third embodiment is nowdescribed with reference to FIG. 21.

[0097] In the structure of the semiconductor laser device according tothe third embodiment, an n-type contact layer 5, an anti-cracking layer6, an n-type second cladding layer 7, an n-type first cladding layer 8,an MQW emission layer 9, a p-type first cladding layer 10, a p-typesecond cladding layer 11, current blocking layers 12, a p-type contactlayer 13 and protective films 14 are formed on the undoped layer GaNlayer 45 shown in FIG. 20, similarly to the first embodiment. Thecompositions and thicknesses of the layers 5 to 13 and the protectivefilms 14 are similar to those in the first embodiment.

[0098] A p-side electrode 15 is formed on the upper surface of thep-type contact layer 13 while an n-side electrode 16 is formed on apartially removed and exposed surface of the n-type contact layer 5.

[0099] In the semiconductor laser device according to the thirdembodiment, the undoped GaN layer 45 having excellent mass productivityand low dislocation density formed by the method of forming anitride-based semiconductor according to the third embodiment shown inFIG. 15 to 20 is employed as an underlayer for forming the layers 5 to13 thereon as hereinabove described, whereby excellent crystallinity canbe implemented in the layers 5 to 13. Consequently, a semiconductorlaser device having excellent mass productivity and excellent devicecharacteristics can be obtained.

[0100] (Fourth Embodiment)

[0101] Referring to FIGS. 22 to 27, an n-type SiC (0001) plane substrate51 (hereinafter referred to as “SiC substrate 51”) having conductivityis employed in a fourth embodiment of the present invention in place ofthe insulating sapphire substrate 41 employed in the third embodiment. Amethod of forming a nitride semiconductor according to the fourthembodiment is now described in detail with reference to FIGS. 22 to 27.

[0102] First, buffer layers 52 of Si-doped AlGaN having a thickness ofabout 15 nm and an underlayer 53 of Si-doped GaN having a thickness ofabout 2 μm are formed on the n-type SiC substrate 51 by a crystal growthmethod such as MOVPE, as shown in FIG. 22. The SiC substrate 51 is anexample of the “substrate” according to the present invention.

[0103] Striped mask layers 54 of Sio₂ having a thickness of about 0.5 μmare formed on the underlayer 53. The stripe patterns of the mask layers54 are formed in a cycle of about 6 μm so that the width of the masklayers 54 is about 5 μm and the interval between each adjacent pair ofmask layers 54 (the width of mask openings) is about 1 μm. The stripedmask layers 54 are formed in parallel to the [11-20] direction of theunderlayer 53 consisting of Si-doped GaN.

[0104] The mask layers 54 are employed as masks for etching the surfaceof the underlayer 53 by a thickness of about 1 μm through RIE or thelike. Thus, the surface of the underlayer 53 is brought into an unevenshape, as shown in FIG. 23. The shape of the projection portions varieswith etching conditions, such that upper parts of recess portions may belarger or smaller in width than bottom parts thereof. In the followingdescription, projection portions of the etched underlayer 53 are in theform of a mesa (trapezoid). The uneven surface of the underlayer 53 hasridges of a height of about 1 μm, and they are formed in parallel to the[11-20] direction of the underlayer 53 consisting of Si-doped GaN.

[0105] Then, Si-doped GaN layers 55 are re-grown from the bottom andside surfaces, serving as seed crystals, of the exposed recess portionsof the underlayer 53 consisting of Si-doped GaN, as shown in FIG. 24. Inan initial stage, the Si-doped GaN layers 55 are vertically (upwardly)grown from the bottom surfaces of the recess portions of the underlayer53 and also laterally grown from the side surfaces of the recessportions of the underlayer 53, as shown in FIGS. 24 and 25. The Si-dopedGaN layers 55 are examples of the “first nitride-based semiconductorlayer” according to the present invention.

[0106] From the state shown in FIG. 25, the Si-doped GaN layers 55 arelaterally grown on the mask layers 54, as shown in FIG. 26. The Si-dopedGaN layers 55 laterally grown on the mask layers 54 coalesce into acontinuous Si-doped GaN layer 55 of about 5 μm in thickness having aflattened surface, as shown in FIG. 27.

[0107] In the method of forming a nitride-based semiconductor accordingto the fourth embodiment, the Si-doped GaN layers 55 are grown from thebottom and side surfaces, serving as seed crystals, of the recessportions of the underlayer 43 consisting of Si-doped GaN as hereinabovedescribed, whereby dislocations of the Si-doped GaN layers 55 are bentin the in-plane direction of the (0001) plane of the Si-doped GaN layers55 when the Si-doped GaN layers 55 are laterally grown from the sidesurfaces of the recess portions of the underlayer 53 or on the masklayers 54. Thus, the dislocation density can be reduced around thesurfaces of the Si-doped GaN layers 55.

[0108] In the method of forming a nitride-based semiconductor accordingto the fourth embodiment, the surface of the underlayer 53 is broughtinto an uneven shape as hereinabove described, whereby only the surfaceof the underlayer 53 may be etched. Thus, the etching time for formingthe projection portions on the surface on the underlayer 53 can bereduced as compared with the conventional process of forming theprojection portions on the surface shown in FIG. 30. Consequently, amethod of forming a nitride-based semiconductor having excellent massproductivity can be obtained similarly to the first to thirdembodiments.

[0109] In the method of forming a nitride-based semiconductor accordingto the fourth embodiment, the underlayer 53 consisting of Si-doped GaNis grown after forming the buffer layers 52 on the SiC substrate 51,whereby the underlayer 53 having low dislocation density can be readilyformed.

[0110]FIG. 28 is a perspective view showing a semiconductor laser devicefabricated with the aforementioned method of forming a nitride-basedsemiconductor according to the fourth embodiment. The structure of thesemiconductor laser device fabricated with the method of forming anitride-based semiconductor according to the fourth embodiment is nowdescribed with reference to FIG. 28.

[0111] In the structure of the semiconductor laser device according tothe fourth embodiment, an anti-cracking layer 25, an n-type secondcladding layer 26, an n-type first cladding layer 27, an MQW emissionlayer 28, a p-type first cladding layer 29, a p-type second claddinglayer 30, current blocking layers 31 and a p-type contact layer 32 areformed on the Si-doped layer GaN layer 55 shown in FIG. 27, similarly tothe second embodiment. The compositions and thicknesses of the layers 25to 32 are similar to those in the second embodiment.

[0112] A p-side electrode 33 is formed on a projection portion of thep-type contact layer 32 reflecting the mesa shape of the p-type secondcladding layer 30. The SiC substrate 51 has conductivity, and hence ann-side electrode 34 is formed on the back surface of the SiC substrate51.

[0113] In the semiconductor laser device according to the fourthembodiment, the Si-doped GaN layer 55 having excellent mass productivityand low dislocation density formed by the method of forming anitride-based semiconductor according to the fourth embodiment shown inFIG. 22 to 27 is employed as an underlayer for forming the layers 25 to32 thereon as hereinabove described, whereby excellent crystallinity canbe implemented in the layers 25 to 32. Consequently, a semiconductorlaser device having excellent mass productivity and excellent devicecharacteristics can be obtained.

[0114] While the sapphire substrate 41 and the SiC substrate 51 areemployed in the aforementioned third and fourth embodimentsrespectively, for example, the present invention is not restricted tothis but a spinel substrate, a GaN substrate, a GaAs substrate, a GaPsubstrate, an InP substrate, a ZrB₂ substrate or a quartz substrate mayalternatively be employed.

[0115] Although the present invention has been described and illustratedin detail, it is clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the spirit and scope of the present invention being limitedonly by the terms of the appended claims.

[0116] While the recess portions of the surfaces of the sapphiresubstrate 1, the Si substrate 21 and the underlayers 43 and 53 areformed in the height of about 1 m in the aforementioned first to fourthembodiments, the present invention is not restricted to this but theheight of the recess portions is preferably set in the range of severalnm to several μm not reaching the bottom surfaces of the sapphiresubstrate 1., the Si substrate 21 and the underlayers 43 and 53.

[0117] While the striped mask layers 2, 22, 44 and 54 are formed inparallel with the [1-100] direction of the sapphire substrate 1, the[1-10] direction of the Si substrate 21 and the [11-20] directions ofthe GaN underlayers 43 and 53 in the aforementioned first to fourthembodiments respectively, the present invention is not restricted tothis but the striped mask layers may alternatively be formed in adirection different from the aforementioned ones. For example, the masklayers 44 and 45 according to the third and fourth embodiments may beformed in parallel with the [1-100] directions of the GaN underlayers 43and 53.

[0118] While the projection portions of the surfaces of the sapphiresubstrate 1, the Si substrate 21 and the underlayers 43 and 53 areformed in parallel with the [1-100] direction of the sapphire substrate1, the [1-10] direction of the Si substrate 21 and the [11-20]directions of the GaN underlayers 43 and 53 in the first to fourthembodiments respectively, the present invention is not restricted tothis but the projection portions of the surface may be formed in adirection different from the above. For example, the projection portionsof the surfaces of the underlayers 43 and 53 according to the third andfourth embodiments may alternatively be formed in parallel with the[1-100] directions of the GaN underlayers 43 and 53.

[0119] While the mask layers 2, 22, 44 and 54 and the openings thereofare formed in a striped shape in the aforementioned first to fourthembodiments, the present invention is not restricted to this but themask layers may alternatively be formed in a circular, hexagonal ortriangular shape, and the openings thereof may also be formed in acircular, hexagonal or triangular shape. When the mask layers and theopenings thereof are formed in a hexagonal or triangular shape, thesides of the hexagons or triangles may match with any crystalorientation.

[0120] While the recess and projection portions of the surfaces of thesapphire substrate 1, the Si substrate 21 and the underlayers 43 and 53are formed in a striped shape in the aforementioned first to fourthembodiments, the present invention is not restricted to this but therecess portions or the projection portions of the surfaces of thesapphire substrate 1, the Si substrate 21 and the underlayers 43 and 53may alternatively be formed in a circular, hexagonal or triangularshape. When the recess or projection potions are formed in a hexagonalor triangular shape, the sides of the hexagons or triangles may matchwith any crystal orientation.

[0121] While the nitride-based semiconductors are employed for preparingsemiconductor laser devices in the aforementioned first to fourthembodiments, the present invention is not restricted to this but alsoapplicable to another device such as a light emitting diode device or atransistor employing a nitride-based semiconductor.

[0122] In each of the aforementioned first to fourth embodiments, thenitride-based semiconductor may have a wurtzite crystal structure or azinc blende crystal structure.

[0123] While crystal growth of each nitride-based semiconductor layer isperformed by MOVPE in the aforementioned first to fourth embodiments,the present invention is not restricted to this but crystal growth mayalternatively be performed by HVPE or gas source MBE employing TMAl,TMGa, TMIn, NH₃, SiH or Cp₂Mg as source gas.

[0124] In each of the first to fourth embodiments, the bottom surfacesof the recess portions of the surface of the sapphire substrate 1, theSi substrate 21 or the underlayer 43 or 53 are preferably formed in awidth within the range of several 100 nm to several 10 μM.

[0125] While the n-type first cladding layers 8 or 27 and the p-typefirst cladding layers 10 or 29 consist of GaN in the aforementionedfirst to fourth embodiments, the present invention is not restricted tothis but the first cladding layers may consist of other materials havinga wider bandgap than the MQW emission layer. For example, AlGaN such asAl_(0.01)Ga_(0.99)N, InGaN such as In_(00.1)Ga_(0.99)N, or AlGaInN suchas Al_(0.01)Ga_(0.98)In_(0.01)N may be employed as the materialsconstituting the first cladding layers.

What is claimed is:
 1. A nitride-based semiconductor element comprising:a substrate comprising a surface having projection portions; a masklayer formed to be in contact with only said projection portions of saidsurface of said substrate; a first nitride-based semiconductor layerformed on recess portions of said substrate and said mask layer; and anitride-based semiconductor element layer, formed on said firstnitride-based semiconductor layer, having an element region.
 2. Thenitride-based semiconductor element according to claim 1, wherein saidsubstrate includes a substrate selected from a group consisting of asapphire substrate, a spinel substrate, an Si substrate, an SiCsubstrate, a GaN substrate, a GaAs substrate, a GaP substrate, an InPsubstrate, a ZrB₂ substrate and a quartz substrate.
 3. The nitride-basedsemiconductor element according to claim 2, wherein said substrateincludes a sapphire substrate, and said mask layer and said projectionportions of said surface of said substrate are formed in the shape ofstripes being parallel to the [1-100] direction of said sapphiresubstrate.
 4. The nitride-based semiconductor element according to claim2, wherein said substrate includes an Si substrate, and said mask layerand said projection portions of said surface of said substrate areformed in the shape of stripes being parallel to the [1-10] direction ofsaid Si substrate.
 5. The nitride-based semiconductor element accordingto claim 1, further comprising a buffer layer formed on the interfacebetween said recess portions of said substrate and said firstnitride-based semiconductor layer.
 6. A nitride-based semiconductorelement comprising: an underlayer, formed on a substrate, consisting ofa nitride-based semiconductor and comprising a surface having projectionportions; a mask layer formed to be in contact with only said projectionportions of said surface of said underlayer; a first nitride-basedsemiconductor layer formed on recess portions of said underlayer andsaid mask layer; and a nitride-based semiconductor element layer, formedon said first nitride-based semiconductor layer, having an elementregion.
 7. The nitride-based semiconductor element according to claim 6,further comprising a buffer layer formed between said substrate and saidunderlayer.
 8. The nitride-based semiconductor element according toclaim 6, wherein said substrate includes a substrate selected from agroup consisting of a sapphire substrate, a spinel substrate, an Sisubstrate, an SIC substrate, a GaAs substrate, a GaP substrate, an InPsubstrate, a ZrB₂ substrate and a quartz substrate.
 9. The nitride-basedsemiconductor element according to claim 6, wherein said underlayerincludes a GaN layer, and said mask layer and said projection portionsof said surface of said underlayer are formed in the shape of stripesbeing parallel to the [11-20] direction or the [1-100] direction of saidGaN layer.
 10. A method of forming a nitride-based semiconductorcomprising steps of: forming projection portions on a surface on asubstrate; forming a mask layer to be in contact with only saidprojection portions of said surface of said substrate; and growing afirst nitride-based semiconductor layer on recess portions of saidsubstrate and said mask layer through said mask layer.
 11. The method offorming a nitride-based semiconductor according to claim 10, furthercomprising a step of forming a buffer layer on said recess portions ofsaid substrate in advance of said step of growing said firstnitride-based semiconductor layer.
 12. The method of forming anitride-based semiconductor according to claim 10, wherein said steps offorming said projection portions on said surface on said substrate andforming said mask layer include a step of forming said mask layer on thesurface of said substrate and thereafter etching the surface of saidsubstrate through said mask layer thereby simultaneously forming saidprojection portions on said surface of said substrate and said masklayer coming into contact with only said projection portions of saidsurface.
 13. The method of forming a nitride-based semiconductoraccording to claim 10, further comprising a step of growing anitride-based semiconductor element layer having an element region onsaid first nitride-based semiconductor layer.
 14. The method of forminga nitride-based semiconductor according to claim 10, wherein saidsubstrate includes a substrate selected from a group consisting of asapphire substrate, a spinel substrate, an Si substrate, an SiCsubstrate, a GaN substrate, a GaAs substrate, a GaP substrate, an InPsubstrate, a ZrB₂ substrate and a quartz substrate.
 15. The method offorming a nitride-based semiconductor according to claim 14, whereinsaid substrate includes a sapphire substrate, and said mask layer andsaid projection portions of said surface of said substrate are formed inthe shape of stripes being parallel to the [1-100] direction of saidsapphire substrate.
 16. The method of forming a nitride-basedsemiconductor according to claim 14, wherein said substrate includes anSi substrate, and said mask layer and said projection portions of saidsurface of said substrate are formed in the shape of stripes beingparallel to the [1-10] direction of said Si substrate.
 17. A method offorming a nitride-based semiconductor comprising steps of: forming anunderlayer consisting of a nitride-based semiconductor on a substrate;forming projection portions on a surface on said underlayer; forming amask layer to be in contact with only said projection portions of saidsurface of said underlayer; and growing a first nitride-basedsemiconductor layer on recess portions of said underlayer and said masklayer through said mask layer.
 18. The method of forming a nitride-basedsemiconductor according to claim 17, further comprising a step offorming a buffer layer on said substrate in advance of said step offorming said underlayer consisting of a nitride-based semiconductor. 19.The method of forming a nitride-based semiconductor according to claim17, wherein said steps of forming said projection portions on saidsurface on said underlayer and forming said mask layer include a step offorming said mask layer on the surface of said underlayer and thereafteretching the surface of said underlayer through said mask layer therebysimultaneously forming said projection portions on said surface of saidunderlayer and said mask layer coming into contact with only saidprojection portions of said surface.
 20. The method of forming anitride-based semiconductor according to claim 17, further comprising astep of growing a nitride-based semiconductor element layer having anelement region on said first nitride-based semiconductor layer.
 21. Themethod of forming a nitride-based semiconductor according to claim 17,wherein said substrate includes a substrate selected from a groupconsisting of a sapphire substrate, a spinel substrate, an Si substrate,an SiC substrate, a GaAs substrate, a GaP substrate, an InP substrate, aZrB₂ substrate and a quartz substrate.
 22. The method of forming anitride-based semiconductor according to claim 17, wherein saidunderlayer includes a GaN layer, and said mask layer and saiLdprojection portions of said surface of said underlayer are formed in theshape of stripes being parallel to the [11-20] direction or the [1-100]direction of said GaN layer.